In recent years, with high integration and high output of electronic materials, substrates with high heat radiation (high thermal conductivity) have been in high demand, which can be replacing alumina substrates that have been used thus far. In consideration of the foregoing situation, a sintered body obtained by adding a sintering auxiliary agent such as berylia and the like to silicon carbide, aluminum nitride and the like, and calcining the resulting mixture is said to be a suitable material satisfying the aforementioned demand. In particular, a sintered aluminum nitride has characteristics such as low toxicity, high insulating property and the like so that such a sintered body has been the most paid attention as a high conductive substrate material.
The sintered aluminum nitride is usually produced by calcining an aluminum nitride powder. The physical properties and chemical properties of the aluminum nitride powder as a raw material control very important properties (density, thermal conductivity and the like) in using the obtained sintered body as a high conductive substrate material. Accordingly, several methods for producing an aluminum nitride powder have been studied and proposed.
For example, a method comprising heating a metallic aluminum in a nitrogen atmosphere or an ammonia atmosphere can be cited. However, this method has a drawback in that since the aluminum nitride powder is obtained as a bulk product that is conspicuously consolidated, it cannot be used as a raw material for calcining.
Furthermore, a method comprising mixing an alumina powder with a carbon powder and heating the resulting mixture in a nitrogen atmosphere or an ammonia atmosphere can be cited. However, in this method, the diameter of alumina raw material, the amount of impurities therein and the like have influence on the properties of products so that highly pure alumina with a fine particle diameter is inevitably used, thus resulting in increasing the cost.
Furthermore, a method comprising applying a heat treatment to a reaction product of an organic aluminum compound and amines can be cited. However, in this method, carbon easily remains in the aluminum nitride powder to be obtained and such carbon has a bad influence on the properties of the finally obtained sintered body.
Furthermore, a method comprising reacting ammonia gas with aluminum chloride and/or aluminum bromide gas in a vapor phase can be cited. However, in this method, hydrogen halide gas as a by-product is generated; the generated gas corrodes the apparatus so that a treatment device for discharging the gas out to the reactive system is needed.
Meanwhile, a method comprising mixing organic aluminum compound gas with ammonia gas at not more than 200° C. and carrying out the vapor phase reaction at from 600 to 1300° C. to produce the aluminum nitride powder can be cited (for example, refer to Patent Document 1).
However, the aluminum nitride powder obtained by the foregoing methods including the method described in Patent Document 1 has a hard sintering property as it is that is originally owned by a substance called aluminum nitride. So, there is a drawback in that a calcination temperature of about 2000° C. is needed so that a special calcining furnace is required, resulting in making the cost of the production facilities higher and increasing the amount of energy used for the production.
For this reason, in calcining aluminum nitride, a method comprising adding a sintering auxiliary agent is generally adopted. This method utilizes the fact that the melting point of a complex oxide of a component contained in the sintering auxiliary agent with aluminum is lower than the melting point (not less than 2000° C.) of the aluminum nitride. Namely, the complex oxide of the sintering auxiliary agent generated during calcination is melted to form a liquid phase and the substance movement (a sintering phenomenon) proceeds by way of the liquid phase. Accordingly, for example, when Y2O3 is used as a sintering auxiliary agent, 1780° C. that is the melting point of a complex oxide of yttrium with aluminum becomes the lower limit of the calcination temperature.
Furthermore, there has been reported that 1600° C. can be the calcination temperature of aluminum nitride by using LiO2—Y2O3—CaO as a sintering auxiliary agent (for example, refer to Non-patent Document 1). However, the foregoing sintering auxiliary agent is special so that its application might be limited by adding alkali metal species.
On the other hand, the person in the field can naturally conceive an idea of making the particle diameter small in order to enhance the sintering property of an inorganic particle. However, the smaller the particle diameter, the higher the sintering property is. On the other hand, however, the cohesive force is increased so that it is difficult to produce an aluminum nitride powder with a primary particle diameter of not more than sub-micron and having small cohesion at the same time.
Under these circumstances, there has been proposed a method for producing an aluminum nitride powder which can be turned into a sintered body at the calcination temperature of from 1600 to 1700° C. by finely crushing the coarse aluminum nitride powder having an average particle diameter of from 1 to 20 μm, the amount of oxygen of not more than 2 weight % and the amount of metal impurities excluding aluminum in a non-oxidative atmosphere (for example, refer to Patent Document 2). There has been surely described that an aluminum nitride powder which is sinterable at from 1600 to 1700° C. is obtained by the method described in the foregoing document, in Patent Document 2. However, the properties such as the density, thermal conductivity of the obtained sintered body and the like are not described in detail and it is not clear whether the sintered aluminum nitride which can be properly used actually as a substrate material is obtained.
Furthermore, there has been proposed a method for calcining an aluminum nitride ultrafine particle having an average diameter converted from the specific surface area of 0.06 μm with yttrium fluoride, i.e., a low melting point sintering auxiliary agent added thereto at 1500° C. (for example, refer to Patent Document 3). However, in this method, it is required to use a low melting point sintering auxiliary agent.
Furthermore, in the conventional technique, even though the minimum of 1600° C. is needed as the calcination temperature, when the calcination can be made in the temperature range of lower than 1600° C., a special calcining furnace is no longer needed and the sintered body can be produced with much cheaper cost. So, an aluminum nitride powder which enhances the sintering property and can be turned into a sintered body at a lower calcination temperature than in the past is in demand.                Patent Document 1: JP1988-60102A        Non-patent Document 1: Ceramics Japan (32) N. 6 (1997), published by The Ceramic Society of Japan        Patent Document 2: JP1994-015404B        Patent Document 3: JP1994-211577A        